In many machines, seals are provided between rotating and stationary components. For example, in steam turbines, it is customary to employ a plurality of arcuate seal ring segments to form a labyrinth seal about and between the stationary and rotating components. Typically, the arcuate seal ring segments are disposed in an annular groove in the stationary component designed to be concentric about the axis of rotation of the machine and hence concentric to the sealing surface of the rotating component. Each arcuate seal segment carries an arcuate seal face in opposition to the sealing surface of the rotating component. In labyrinth-type seals, the seal faces carry a radially-directed array of axially spaced teeth, and which teeth are radially spaced from an array of axially spaced annular grooves forming the sealing surface of the rotating component. Alternatively, the rotating component may have a smooth surface in radial opposition to the array of teeth on the seal faces. In any event, the sealing function is achieved by creating turbulent flow of a working media, for example, steam, as it passes through the relatively tight clearances within the labyrinth defined by the seal face teeth and the opposing surface of the rotating component.
In a typical installation, the annular groove is dovetail-shaped, having locating flanges directed axially toward one another and defining a slot therebetween. The stationary component is split lengthwise such that the semi-annular dovetail grooves may receive correspondingly-shaped arcuate seal ring segments. More particularly, the arcuate segments are similarly dovetail-shaped, having a pair of flanges directed axially away from one another for disposition within the dovetail groove and a neck joining the seal face and the flanges of the segment and passing through the slot defined by the locating flanges of the grooves. The neck carries the arcuate seal face radially inwardly of the groove when installed.
In this type of seal, the ability to maintain a tight uniform clearance without physical contact between the rotating and stationary components is critical to the formation of an effective seal. If this radial clearance between the seal faces of the segments and the opposing seal surfaces of the rotating component becomes too large, less turbulence is produced and the sealing action is compromised. Conversely, if the clearance is too tight, the sealing teeth may contact the rotating element, with the result that the teeth lose their sharp profile and tight clearance and thereafter create less turbulence, likewise compromising the sealing action.
Seals of this type often do not obtain the designed uniform radial clearance about and between the stationary and rotating components for a number of reasons. For example, the locating fits, i.e., the locating flanges of the stationary component, may be distorted or out-of-round, relative to the sealing surface of the rotor. Conversely, the locating fits may be perfectly round but lie eccentric to the sealing surface and axis of the rotary component. The radial clearance between the stationary seal faces and the sealing surface of the rotating component, in either case, will not therefore be uniform about the rotor axis. Thirdly, the locating fit may be a custom size or non-standard in size. Even though round, it may not provide the designed radial clearance between the stationary and rotating components when the seal ring segments are replaced. Of course, various combinations of out-of-roundness, eccentricity and non-standard sizes may occur.
Turning first to the locating fits which have become distorted or out-of-round, e.g., as a result of high pressure and temperature applications, such distortion directly affects the dimensional consistency of the labyrinth seal's internal clearance and thus the seal's effectiveness. In most instances, the locating flanges will distort, with an opening effect on one axis which will result in a closing effect on a perpendicular axis. For example, the locating flanges on the stationary component tend to be deformed into an elliptical configuration which prevents the formation of a uniform seal clearance about the rotating component. The magnitude of the ellipticity present translates directly into excessive clearance of the seal faces of the segments relative to the sealing surface of the rotor across the major diameter of the elliptical bore and minimum clearance across the minor diameter of the elliptical bore. The seal clearances thus vary from a condition of interference on one axis and excessive clearance on the other, resulting in loss of the effectiveness of the seal.
As noted previously, the locating fits may themselves be round, but, due to variations of alignment, they may not lie concentric to the sealing surface of the rotating component. As a consequence, the seal faces of the segments may lie eccentric to the locating fit, i.e., the locating flanges, but do not afford a uniform radial clearance between the stationary seal faces and the sealing surface of the rotating component about the full circumference of those components. Also, where the locating flanges are non-standard or of unknown dimension due to prior field machining, standard seal segments may not provide the designed uniform radial clearance even if the locating flanges are perfectly round. The seal clearances must be maintained substantially uniform and at the designed clearance.
When renewing labyrinth seals of this type after use, the design of the locating flanges of the groove into which the arcuate segments fit prevent their ready adjustment in radial location. Restoration of the locating flanges of the groove is too costly. Typically, when new arcuate seal segments were installed into a groove where the locating flanges are distorted or out-of-round, or eccentric, the installer carefully identified the locations where the seal clearance was too tight. Once identified, the installer would hand-scrape or grind the edges of the seal teeth to produce the necessary clearance. No remedial action was typically taken where the seal tooth-to-rotor clearance was excessive. This excessive clearance was conventionally left in an as-is condition. Because of the time and expense involved in the restoration of the locating flanges of the groove to true concentricity about the rotor surface, excessive seal clearances were conventionally accepted as a parasitic loss associated with age and distortion, and not repaired.
In U.S. patent application Ser. No. 07/981,520, filed Nov. 25, 1992, of common assignee herewith, there is illustrated apparatus and methods for establishing a substantially uniform radial clearance about and between the rotating component and the segment seals. This is accomplished by providing locating elements along the locating flanges of the arcuate segments which could be adjusted or ground to an appropriate depth to adjust the clearance between the locating flanges of the segment and the groove, thereby to maintain concentricity of the stationary and rotating seal faces, while simultaneously mounting the arcuate segments directly on the distorted, eccentric or non-standard size locating flanges of the groove. In practice, axially extending dowels have been employed along the underside of the locating flanges of the arcuate segments and which dowels are typically ground in the field when the segments are to be replaced to assure this concentricity. Use of these round dowels in the field during installation of the arcuate segments requires substantial installation time and expense because of the required machining and replacement of the existing dowels. Machining dowels in place to adjust the clearances is very difficult and remaking new dowels when shy of stock material is extremely time-consuming.